1.3 Organic Molecules in Living Organisms

Carbon provides the structural framework of biological molecules because a carbon atom has four electrons available for covalent bonding and can complete its outer shell by sharing four more. It can make four single bonds, two double bonds, a triple bond together with a single bond, or other combinations that give a total bonding capacity of four. Carbon also bonds strongly to carbon. Repeating carbon–carbon bonds create chains, branched skeletons and rings that remain stable under the aqueous, moderate-temperature conditions of cells.

The four bonding directions around a carbon atom are arranged in three dimensions rather than on a flat cross. This geometry gives molecules definite shapes. Adding double bonds restricts rotation, while branching and ring formation alter the relative positions of functional groups. Two compounds can consequently have the same molecular formula but different arrangements of atoms. Such structural isomers may differ in solubility, reactivity and recognition by enzymes because biological interactions depend on three-dimensional fit as well as elemental composition.

Carbon skeletons alone are largely non-polar. Their biological behavior changes when other atoms are attached. Oxygen often introduces polarity through hydroxyl or carbonyl groups; nitrogen appears in amino groups and nitrogenous bases; phosphorus occurs in negatively charged phosphate groups; sulfur can form covalent links between parts of a protein. A molecule's properties therefore arise from the carbon framework together with the identity, position and orientation of its functional groups.

Carbohydrates usually contain carbon, hydrogen and oxygen, often close to the empirical pattern (CH₂O)ₙ. Monosaccharides such as glucose are small carbohydrate monomers. Two monosaccharides can form a disaccharide, and many can form a polysaccharide. Carbohydrates provide respiratory substrates and short- to medium-term energy stores; cellulose also provides tensile structural support in plant cell walls. The arrangement of glycosidic bonds, rather than the mere presence of glucose, determines whether a polymer is compact, branched, digestible or load-bearing.

Proteins are built from amino acids. Every amino acid used in translation has an amino group, a carboxyl group, a hydrogen atom and a variable R group attached to a central carbon. The R group distinguishes one amino acid from another. Amino acids join into polypeptides, and the sequence then folds into a specific structure. Proteins can catalyse reactions, transport substances, transmit signals, bind antigens, generate movement and form strong fibres. These roles depend on sequence and folding, not simply on protein mass.

Lipids are a chemically diverse category rather than one repeating polymer family. Triglycerides contain glycerol joined to three fatty acids, phospholipids contain a polar phosphate-bearing head and non-polar tails, and steroids contain four fused carbon rings. Their high proportion of carbon–hydrogen bonds makes many lipids energy-rich and non-polar. Insolubility in water makes triglycerides osmotically suitable stores, while the amphipathic nature of phospholipids causes them to form membrane bilayers.

Nucleic acids are polymers of nucleotides. Each nucleotide contains a five-carbon sugar, a phosphate group and a nitrogenous base. The sugar–phosphate backbone gives the chain direction and stability; variation in base sequence stores information. DNA preserves hereditary information and RNA has several roles in expressing it. Nucleotides also occur outside nucleic acids: ATP transfers energy through phosphate-group reactions, and related nucleotides participate in electron transfer and cell signalling.

A monomer is a relatively small molecule capable of becoming a repeating or related subunit in a larger structure. A polymer is a large molecule made by covalently linking many monomers. The language must be applied carefully. Monosaccharides form polysaccharides, amino acids form polypeptides, and nucleotides form nucleic acids. Triglycerides are assembled from glycerol and fatty acids but are not polymers, because they do not consist of a long chain of repeating monomer units.

A condensation reaction joins two molecules and releases water. In carbohydrate synthesis, hydroxyl groups participate in formation of a glycosidic bond. In protein synthesis, the carboxyl group of one amino acid reacts with the amino group of another to create a peptide bond. Glycerol's three hydroxyl groups can react with the carboxyl groups of three fatty acids to create three ester bonds. Nucleotides form phosphodiester bonds between sugar and phosphate. The bond and enzyme differ among molecule classes, but the shared bookkeeping rule is that covalent joining is accompanied by water formation.

Condensation is anabolic because it builds larger molecules from smaller ones and usually belongs to energy-requiring biosynthetic pathways. The reaction is not caused merely by removing water from a cell; enzymes orient particular functional groups and couple bond formation to suitable energy sources. If a linear chain contains n monomers, joining them requires n − 1 links and therefore produces n − 1 water molecules. A branched polymer follows the same logic: every new covalent link between previously separate subunits increases the connected structure by one subunit.

Hydrolysis reverses this molecular accounting. Water is split, and its components are added across a covalent bond as the larger molecule separates. Digestive enzymes hydrolyse starch to smaller sugars, proteins to peptides and amino acids, and triglycerides to fatty acids and glycerol. Hydrolysis is catabolic in these cases, but catabolism should not be defined as simply 'releasing energy': it means breaking complex molecules into simpler ones. Some hydrolytic steps require energy elsewhere in a pathway even though the overall catabolic process may release usable energy.

A functional group is a recognizable group of atoms that gives similar chemical behavior to the different molecules in which it occurs. The carboxyl group, written –COOH, contains a carbon double-bonded to oxygen and single-bonded to a hydroxyl group. It is polar and can donate a hydrogen ion, so at cellular pH many carboxyl groups occur in the negatively charged –COO⁻ form. Carboxyl groups are present in fatty acids and amino acids and participate directly in formation of ester and peptide bonds.

An amino group contains nitrogen and is often written –NH₂. Because the nitrogen has a lone pair of electrons, amino groups can accept a hydrogen ion and commonly occur as –NH₃⁺ under biological conditions. Every standard amino acid contains an amino group. During peptide-bond formation, the amino group of one amino acid reacts with the carboxyl group of another. The charges of amino and carboxyl groups also affect solubility, acid–base behavior and the ionic interactions that stabilize folded proteins.

A phosphate group contains phosphorus bonded to oxygen atoms and carries negative charge in cells. Phosphate groups make nucleotides hydrophilic, form the links in nucleic-acid backbones and provide the polar heads of phospholipids. Transfer of a phosphate group from ATP can change a protein's activity by altering charge and conformation; removal by dephosphorylation can reverse the change. Phosphate ions also contribute to buffering, resisting sudden pH changes, and phosphorus availability can limit productivity in freshwater ecosystems.

Functional groups offer a better explanation than labels alone. A fatty acid is amphipathic at molecular scale because its carboxyl end is polar while its long hydrocarbon region is non-polar. An amino acid can carry both positive and negative regions. A nucleotide combines a charged phosphate, a polar sugar and a relatively hydrophobic base. In an exam response, identifying these regions and their interactions with water often explains solubility, orientation, bonding and location more precisely than saying a molecule is simply organic.

Hydroxyl groups, written –OH, are another recurring feature. They make sugars and glycerol polar and offer sites for condensation. A carbonyl group contains C=O and occurs as an aldehyde at a chain end or a ketone within a chain; its position helps distinguish monosaccharide isomers. A methyl group, –CH₃, is non-polar and can change molecular recognition when added to DNA or protein surfaces.

Molecular drawings use conventions that must be read actively. A displayed formula shows every relevant atom and bond, a structural formula groups atoms while preserving connectivity, and a skeletal formula omits most carbon and hydrogen labels. Wedges and dashed bonds can indicate groups projecting toward or away from the viewer. None is a photograph of a molecule; each model emphasizes selected information.

Scale also changes the appropriate model. Bond lengths are measured in nanometres, cells in micrometres and organismal structures in millimetres or metres. One nanometre is 10⁻⁹ m and one micrometre is 10⁻⁶ m. Keeping prefixes explicit prevents thousand-fold errors and helps connect molecular dimensions to membrane thickness, organelle size and microscope resolution.